1. Field of the Invention
This invention relates generally to lighting systems, having dimming capabilities, wherein light is derived from individual lamps, or groups of lamps, adapted to operate over a wide range of selected levels of brightness.
2. Description of the Prior Art
It is often desirable to vary the level of illumination emitted from incandescent lamps used in theaters and television studios. One simple solution is to employ a rheostat in series with a number of mono-filament lamps, but a disadvantage of this method is the excessive waste of power by the rheostat at low light level settings. Variable autotransformers, while more efficient, still occupy considerable space and, like rheostats, require mechanical coupling means. Another system involves the use of a reactance dimmer, wherein the A.C. line which energizes the lamps is serially connected to a magnetic core winding. A D.C. control current is allowed to flow through another winding on the same core, thus varying the reactance presented to the A.C. lamp current.
The development of thyristor devices gave rise to another popular method of incandescent lamp brightness control. Two SCR's connected "back to back" (or one Triac) are coupled in series with the lamps to permit load current to flow over a selected portion of both halves of the A.C. cycle when the appropriate gate terminals are energized. The net result is that the average voltage applied to the lamps is varied in a more efficient and space-saving manner, and the power dissipated in the SCR's is minimal, even at low light-level settings.
All of the above-mentioned systems, however, operate to change light intensity by varying the average voltage applied to the lamp filaments. This inherently produces changes in color characteristics of the light in that the color temperature of the lamp changes; that is, the peak frequency of light from the lamp varies with variations in temperature of the lamp filament, and such temperature variations result from changing the supply voltage to the filament. These color temperature changes are objectionable in many applications, especially when illuminating subjects for color television broadcasting. Standard multifilament lamps, commonly used in the home, maintain a substantially constant color temperature when either or both filament sections are energized at full operating voltage. However, gradual brightness control over a wide light-level range is not obtainable because the filament wattage combinations are in low ranges, such as 30, 70, 100 or 25, 50, 75.
Furthermore, even though an SCR-type control circuit is more efficient than earlier rheostat dimmers, its efficiency is somewhat reduced due to its requirement for a large and expensive choke. The choke is necessary for slowing down the rise time of the load current from a few micro-seconds to approximately 500 micro-seconds in order to reduce radio-frequency-interference signals to an acceptable level, and in order to prevent an audible noise from being generated by the lamp filaments due to the steep current rise time. Also, because the SCR controlled devices must be connected in series with the lamps according to the conventional systems, it is necessary to run heavy power wire throughout the entire system. That is, it is necessary to run power wire from the power source to the dimmer bank and then to the lamps, and since the dimmer bank may be located at a substantial distance from the lighted area, extensive lengths of power wire are required.
Alternatively, if the SCR dimmer circuit is mounted on the lamp fixture, the additional power wire is not required, but the choke itself produces a sound which is highly objectionable. On the other hand, if the electronic switching devices are controlled to switch near the zero current level, whereby dimming is effected by completely eliminating selected half-cycles of the supplied power, the choke can be omitted, but a noticeable flickering occurs.